Gun ammunition composed of encapsulated monopropellants

ABSTRACT

MICROCAPSULES OF LIQUID MONOPROPELLANTS PROVIDE PROPELLANT CHARGES FOR GUN AMMUNITION. A CAPSULE WALL MATERIAL MAY BE CHOSEN THAT WILL CONTRIBUTE TO THE ENERGY OF THE CHARGE; AND HIGH LOADING DENSITY OF PROPELLANT MAY BE ACHIEVED BY THE USE OF CAPSULES HAVING CAREFULLY SELECTED BIMODAL SIZES.

July 31, 1973 E. G. DORSEY, JR, ET AL 3,749,615

GUN AMMUNITION COMPOSED OF ENCAPSULATED MONOPROPELJIJANT Filed June 11, 1971 lllI- ill! I I N VENTORS [aware 6T 00/5254 Jr Jo/m Q flbfe/"sa/z P0$erf K 111/70 AWE/V7 United States Patent 3,749,615 GUN Ah/MUNTTION COMPOSED OF ENCAPSU- LATED MONOPROPELLANTS Edward G. Dorsey, In, John A. Peterson, and Robert K.

lLuud, Brigham City, Utah, assiguors to Thiokol Chemical Corporation, Bristol, Pa.

Filed June 11, 1971, Ser. No. 152,205 Int. Cl. C06d /08, 5/10 US. Cl. 149-6 11 Claims ABSTRACT OF THE DISCLOSURE Microcapsules of liquid monopropellants provide propellant charges for gun ammunition. A capsule wall material may be chosen that will contribute to the energy of the charge; and high loading density of propellant may be achieved by the use of capsules having carefully selected bimodal sizes.

BACKGROUND OF THE INVENTION This invention relates to gun propellants, and particularly to those having encapsulated, liquid fuels. The invention herein described was made in the course of or under a contract with the US. Air Force.

Liquid propellants have characteristics that yield several advantages over conventional solid propellants in guns, such as low flame temperature, reduced smoke and flash, reduced fouling of gun parts, and longer tube life for the gun barrel. Rapid fire weapons, especially, have notoriously short barrel lives. These anticipated advantages prompted extensive experimentation wherein liquid monopropellants were loaded directly into cartridge cases, and variables such as ignition parameters, propellant compositions and geometric factors were systematically investigated. These experiments proved that the above advantages are indeed available in liquid propellants. Fouling of gun parts, production of smoke, and muzzle flash were reduced significantly. Also, bore erosion was greatly reduced because of the lower flame temperature and absence of erosive particles in liquid propellants.

However, instantaneous burning surface areas in bulk loaded liquid propellants are unpredictable. Vegaries such as Taylor instabilities, wherein surface disturbances may grow exponentially in the liquid, the Helmholtz instabilities, that result from the passage of gas bubbles through the liquid, have been predicted theoretically and verified experimentally. Such turbulences may cause high, erratic pressures accompanied by high frequency, high amplitude pressure excursions. This action damages guns, causes erratic behavior of gas operated automatic weapons, and produces variations in muzzle velocities. Although the fluid dynamics of the system wherein a liquid monopropellant is accelerated and consumed by its own combustion gases have been treated theoretically, the instabilities that characterize the process and the many variables that are not analytically tractable indicate that bulk liquid propellants cannot be easily controlled. Extensive experimentation has failed to solve this problem in use of bulk loaded, liquid propellants.

SUMMARY OF THE INVENTION The present invention, which solves this difficulty in the use of liquid propellants in gun ammunition, is essentially the microencapsulation of liquid monopropellants, which may comprise propellant charges for gun ammunition.

Encapsulated liquid propellants containing two hypergolic components, such as a fuel and and oxidizer encapsulated in separate capsules, are taught in US. Pat. 2,960,935 to D. A. Colpitts. In this patent they are intended for use in rocket igniters. Large, elongated capsules are arranged geometrically in a tubular housing so that each capsule of fuel is adjacent a capsule of oxidizer. When an appropriately severe shock is applied, the capsules are ruptured and the hypergolic liquids are mixed. Although this invention is suitable for its intended use in rocket igniters, it would have a number of disadvantages as propellant for gun ammunition. Large capsules would individually produce the same erratic liquid burning surfaces that create unpredictable pressures in bulk loaded liquid propellants as described above. Although varying pressures are tolerable within limits in rocket igniters, they are very objectionable in gun ammunition, wherein the range and velocity of a projectile must be predictable with considerable precision. Also, the configuration and arrangement of the capsules of the Colpitts patent do not afford a sufficiently high loading density of propellant in the tubular housing to be useful in gun ammunition.

For these reasons, the present invention uses microcapsules, which may be used in sufficiently large numbers that their burning surfaces average out to produce predictable pressures and impart replicable velocities to the projectiles. Monopropellants are used in the present invention to insure uniform burning; since it would be virtually impossible to be certain that microcapsules containing two difierent propellant components are uniformly mixed in a cartridge case.

Greater propellant loading density is achieved in the present invention by the use of capsules of two different sizes, so that the smaller capsules tend to fill the interstices between the larger capsules. Other interstitial fills may be used for special applications.

US. Pat. 3,441,455 teaches encapsulation of solid propellant components and cites materials useful in the present invention. However, it is primarily concerned with preventing particles of solid, chemically active ingredients from reacting with the solid propellant matrix in which they are embedded. Hence, it does not suggest the primary features of the present invention, such as liquids, monopropellants, encapsulation for gun ammunition, or bimodal size distribution of the capsules.

Objects of the present invention are to provide a controlled means for using liquid propellants in gun ammunition, thereby realizing the benefits of low flame temperatures, smooth pressures, and absence of erosive particles; and to achieve maximum loading density of encapsulated liquid monopropellants in cartridge or shell cases. Important features of the invention are the general simplicity of the manufacturing process, and the use of monopropellants to eliminate the problem of uniform mixing that would be diflicult if two or more components of propellant were encapsulated. Another important feature of the invention is that it can produce standard projectile velocities using conventional shell and gun equipment designs.

These and other objects and advantages of the invention will become more apparent as the following detailed description of the invention is read with reference to the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying figure is an enlarged side elevation of a typical rifle shell, with parts broken away to show the propellant of the invention installed therein. The propellant capsules are enlarged relative to the shell size to clarify the bimodal capsule size distribution.

3 DESCRIPTION OF THE PREFERRED EMBODIMENTS The typical rifle shell shown in the figure comprises a tubular shell case 2, an explosive primer cap 3 in one end thereof, a projectile 4 held in the opposite end of the tubular case 2, and a propellant 5 in the shell case 2 between the primer cap 3 and the projectile 4. The propellant 5 comprises microcapsules 6, each having a 'capsule Wall 7 filled with a monopropellant 8. Interstices 9 between the capsules 6 are filled with interstitial fill material, which in a preferred embodiment of the invention comprises smaller capsules 10 of the same type as the capsules 6.

In the present invention, a preferred monopropellant is in general selected from the alkyl nitrates. Specifically useful is a mixture by weight of 60% ethyl nitrate and 40% normal propyl nitrate. For specific purposes, however, virtually any liquid monopropellant could be used. Other monopropellants that are especially useful are hydrazines and nitromethanes.

It has been found that some advantages in handling and storage life of the resulting propellants may be realized by gelling the monopropellants. Inclusion of approximately 3 to 10% of high molecular weight polyethylene glycol, together with a small amount of curing and crosslinking agents (e.g., toluene diisocyanate and a small amount of benzyl dimethylamine), produces very satisfactory gels when mixed with the alkyl nitrates. Low molecular weight polyethylene glycols have been found not only to produce gels, but also, within limits, they can be used to adjust the burning rates of the monopropellants.

The capsule walls are made of a mixture of a polyvinyl alcohol dissolved in water, carrageenin, and optionally a curing agent such as tris (hydroxymethyl) nitromethane.

The encapsulation process is well known and is not considered to be a part of the present invention. However, two types of equipment were used to prepare the capsules of the present invention.

A centrifugal extrusion device used in the present invention consisted of a rotating head with concentric orifice nozzles directed radially outward from the axis of rotation. Monopropellant pumped into the inner chamber flowed through tubes in the orifices in the periphery of the head, the tubes being smaller than the orifices so that a small annulus of space surrounded each tube. The shell material, in fluid form, was gear pumped into the head and flowed through the annuli. In etfect, this extruded rods of fluid monopropellant encased in sheaths of fluid shell material. These rods broke into individual capsules when being projected to a collection area surrounding the base of the encapsulation apparatus. The extrusion head was about seven feet above the collection area, and the capsules were sufiiciently firm that they did not rupture on reaching the collection sheet.

The encapsulation head, shell material lines, and pump were all heated to about 50 to 60 C. to insure fluidity of the carregeenin, which, though quite fluid when warm, forms a firm gel when cool. Without the carrageenin, higher temperatures and increased amounts of polyvinyl alcohol would be necessary and poor ratios of monopropellant to shell material would result.

This apparatus was used to prepare the larger capsules of the bimodal capsule-size system. The larger capsules ranged in diameter size from about 840 to about 1500 microns, although capsules as large as 2000 microns in diameter were successfully made. The larger capsules were deliberately kept in the microcapsule ranges to promote predictable and controllable burning surfaces of the monopropellants.

The smaller capsules of the bimodal system generally ranged in size from about 50 to about 400 microns in diameter, although some capsules as small as 10 microns in diameter were produced. These smaller capsules were made by a second apparatus, also Well known as an encapsulation device. These capsules Were prepared by gravity feeding an emulsion of alkyl nitrate in an aqueous polymer solution onto the center of a three inch diameter, high speed, air driven, rotating disk. Very small emulsion droplets were projected from the periphery of the disk and fell two feet to a collection area. The resulting small capsules were gelled and almost dry when collected. As in the case of the centrifugal extrusion apparatus, the capsules were collected on a polyethylene film. After collection the capsules were transferred to a fluidized bed for final drying and removal of the starch (which was used to coat the collection film).

The resulting propellant was found to comprise approximately 86.8% monopropellant and about 13.2% capsule wall material. Volumetric loading of the monopropellant was enhanced further by removing the empty and partially filled capsules. This was done by pouring the capsules into a bath of ethyl-propyl nitrate (in which they are insoluble) and skimming off the empty and partially filled capsules that floated to the surface. The remaining capsules were then removed from the bath and dried. An average increase in projectile velocity of 146 feet per second was realized by this procedure.

Other interstitial fill materials may be used instead of the smaller capsules to fill the interstices between the larger capsules. By this means, the total energy of the propellant may also be adjusted as desired. Examples of such interstitial fill material that were used successfully for this purpose are balled smokeless powder of the type used in conventional ammunition, granular nitrocellulose, and granular ammonium nitrate.

The energy of the capsule wall material was optionally enhanced by inclusion therein of up to 14% by weight of nitrocellulose. Alternatively, good results were obtained by spraying the completed capsules with a nitrocellulose solution. This solution was prepared by mixing, in percentages by weight: butyl acetate 50%, isopropyl alcohol 40%, ethyl cellulose 8%, and dioctyl phthalate 2%.

Optimum relative proportions of large to small capsules to achieve maximal loading of propellant per volume is, of course, dependent on the sizes of the capsules. However, this desirable condition is approximated with the capsule size range cited above if the mixture of capsules contains from about 60% to about 70% of the larger capsules. Using specific capsule diameter sizes of 1000 microns and 50 microns, optimal loading can be achieved by a 67.5% and 32.5% loading, respectively. 1

The following specific examples illustrate preparation of monopropellants and capsule wall compositions.

Example I A gelled monopropellant was prepared by mixing, in parts by weight, 40 parts of ethyl nitrate, 40 parts of normal propyl nitrate, 18.6 parts of polyethylene glycol (Carbowax 6000), 3 parts of ethyl cellulose (N-300) dissolved in normal propyl nitrate, 0.7 part of toluene diisocyanate (Hylene TM), and one drop of benzyl dimethylamine per 25 ml. of the mixture. This mixture was thoroughly stirred and was allowed to stand at room temperature for 24 hours. A firm gel was formed in that length of time, and almost no free liquid was observed after two weeks.

Example II Capsule shell material was prepared by mixing a solution in parts by weight, 91.6 parts of water, 7.5 parts of polyvinyl alcohol (Elvanol 46-22), and 0.9 part of carrageenin (Carrageenin HWG). After thorough mixing, this solution was used for the capsule wall material and capsules of monopropellant were formed as described above with very good results.

Example III Another shell material was prepared by mixing a solution, in parts by weight, of 90.6 parts of water, 7.5 parts of polyvinyl alcohol (Elvanol 46-22), 0.9 part of carrageenin (Carrageenin HWG), and one part of tris (hydroxymethyl) nitromethane. After thorough mixing, this solution was used for capsule wall material as described above. Although satisfactory, somewhat more breakage of the resulting capsules was observed than in the preceding example.

Other materials, such as gelatin, were used with moderate success for the basic encapsulation material.

An invention has been described that constitutes an advance in the art of gun ammunition; and, although the preferred embodiments have been described with considerable specificity with regard to detail, it should be noted that such details may be altered somewhat without departing from the scope of the invention as defined in the following claims.

The invention claimed is:

1. Propellant for gun ammunition comprising capsules of liquid monopropellant smaller than 2,000 microns in diameter, wherein the monopropellant is selected from the group consisting of alkyl nitrates, hydrazines, and nitromethanes and wherein the capsule wall is a cured mixture of water, polyvinyl alcohol and carrageenin.

2. The propellant of claim 1 wherein the capsules are of two diameter sizes uniformly mixed, the larger ranging from about '840 to about 1500 microns and the smaller ranging from about 50 to about 400 microns, and the larger capsules comprising from about 60% to about 70% by weight of the mixture, whereby the smaller capsules may fill interstices between the larger capsules to achieve maximal density of propellant per volume.

3. The propellant of claim 1 wherein the liquid monopropellant is selected from the alkyl nitrates.

4. The propellant of claim 1 wherein the liquid monopropellant comprises a mixture of about 60% by weight of ethyl nitrate and about 60% by weight of ethyl nitrate and about 40% of normal propyl nitrate.

6 5. The propellant of claim 3 wherein the liquid monopropellant is gelled.

6. The propellant of claim 5 wherein the gelled liquid monopropellant comprises, in approximate percentages by weight:

Ethyl nitrate 40.0 n-Propyl nitrate 40.0 Polyethylene glycol 18.6

Curing and crosslinking agents 1.4

7. The propellant of claim 1 wherein the capsule walls comprise a mixture of a polyvinyl alcohol, carrageenin, and a polymerizing agent.

8. The propellant of claim 1 wherein the capsule wall comprises, in percentages by weight:

Water 91.6 Polyvinyl alcohol 7.5 Carrageenin 0.9

9. The propellant of claim 6 wherein the capsule walls include up to 14% by weight of the composition of nitrocellulose, whereby the total energy of the propellant may be enhanced on combustion.

10. The propellant of claim 1 wherein the capsules are spray coated with nitrocellulose, whereby the energy thereof may be enhanced on combustion.

11. The propellant of claim 1 further including an interstitial fill material to fill the interstices between the capsules thereof, said material being selected such that its energy output on combustion may be used to adjust the total energy output of the propellant as desired.

References Cited UNITED STATES PATENTS 3,143,446 -8/1964 Berman 1492 BENJAMIN R. PADGETT, Primary Examiner US. Cl. X.R. 

